Design and implementation of a DECT stack for Linux Patrick McHardy <kaber@trash.net> Linux Kongress 2010, Nürnberg http://dect.osmocom.org/
About This presentation will present a DECT stack for Linux, containing all components starting from baseband drivers up to an asterisk channel driver. Started around January 2009 after fixing a few bugs in the dedected.org software and realizing how far away from a full implementation it was. Development was primarily motivated by the desire to fool around with this quite old technology and see how buggy other implementations really are :)
About Initially based on the reverse engineered RX-only Com-on-Air PCMCIA driver from the dedected.org project, but basically fully rewritten since then. Mainly written by myself, except for the DSAA (DECT Standard Authentication Algorithm) and DSC (DECT Standard Cipher), which have both been reverse engineered and implemented by the dedected.org project, who also helped me a lot in getting TX support into the firmware.
What is DECT? ● DECT stands for “Digital Enhanced Cordless Telecommunications”, a short-range wireless communications standard used primarily for cordless telephony, but also for Data Terminals, Wireless payment terminals, Baby monitors, Door openers, Traffic Light control, Industrial control, ... ● Specified by ETSI in 1987, further enhanced since then. ● Quite similar to other wireless telecommunication standards like GSM or TETRA.
DECT standards The DECT standards are split into two parts: DECT Common Interface (CI) contains the base standard, DECT application profiles define requirements for specific applications like telephony, packet radio service, etc. and build on the Common Interface. Even the base standard alone is huge and contains over 1200 pages. ● Common Interface: ETSI EN 300 175-1 – EN 300 175-8
DECT standards Common Interface ● ETSI EN 300 175-1: Part 1: Overview ● ETSI EN 300 175-2: Part 2: Physical layer (PHL) ● ETSI EN 300 175-3: Part 3: Medium Access Control (MAC) layer ● ETSI EN 300 175-4: Part 4: Data Link Control (DLC) layer ● ETSI EN 300 175-5: Part 5: Network (NWK) layer ● ETSI EN 300 175-6: Part 6: Identities and addressing ● ETSI EN 300 175-7: Part 7: Security features ● ETSI EN 300 175-8: Part 8: Speech and audio coding and transmission
DECT standards Profiles This list contains a small excerpt of the most common profiles, there are many more: ● Public Access Profile (PAP): ETSI EN 300 175-9: obsoleted by GAP ● Generic Access Profile (GAP): ETSI EN 300 444
DECT standards Profiles ● NG DECT Part 1: Wideband speech: ETSI TS 102 527-1 ● NG DECT Part 2: transparent IP packet data: ETSI TS 102 527-2 ● NG DECT Part 3: Extended wideband speech services: ETSI TS 102 527-3 ● NG DECT Part 4: Light Data Services, Software Update Over The Air (SUOTA), content downloading and HTTP based applications: ETSI TS 102 527-4
DECT Common Interface The DECT Common Interface contains the following parts: ● Part 1: Overview: overview of the system and protocol architecture, definition of terms ● Part 2: Physical layer (PHL): s pecifies radio parameters (modulation, frequency, timing, power values), TDMA frame structure, packet formats, synchronization, primitives to higher layers.
DECT Common Interface ● Part 3: Medium Access Control (MAC) layer: specifies MAC services (broadcast message control (BMC), connectionless message control (CMC), traffic-bearer control (TBC), multi-bearer control (MBC)), definition of logical channels, various control messages, multiplexing and mapping to physical channels.
DECT Common Interface ● Part 4: Data Link Control (DLC) layer: specifies logical data links, C-plane (control plane) services, U-plane (user plane) services, MAC connection management ● Part 5: Network (NWK) layer: specifies functions for Link Control, Call Control, Mobility Management, Supplementary Services.
DECT Common Interface ● Part 6: Identities and addressing: specifies equipment related identities and their relationships. ● Part 7: Security Features: specifies authentication and ciphering processes, key types and key management, effect of the security features on the lower layers ● Part 8: Speech and audio coding and transmission: specified requirements for real-time speech and other audio services
Basic terminology ● FP (Fixed Part): a DECT base station ● PP (Portable Part): a DECT telephone, terminal, ...
Physical layer Radio spectrum ● DECT operates in the 1880 MHz -1980 MHz band (Europe, Asia, Australia, South America) and 1920 MHz – 1930 MHz band (U.S.). Other bands are defined. ● Carrier width 1.728 MHz, 10 standard carriers, further carriers (up to a total of 64) defined per band.
Physical layer Modulation DECT supports various modulation schemes: ● 2-level modulation mandatory, optionally up to 64- level modulation for higher data rates. ● 1.152 Mbit/s per carrier with 2-level modulation ● 6.912 Mbit/s per carrier with 64-level modulation
Physical layer TDMA DECT uses a TDMA frame structure for access in time: frame of 11520 symbols (bits), split into 24 slots of 480 symbols each, 100 frames per second
Physical layer TDMA slots TDMA slots may be used only partially or adjacent slots may be combined. Defined slot formats are: ● Full slot (480 symbols) ● Half slot (240 symbols) ● Double slot (960 symbols) ● Variable capacity slot j (100+j or 104+j symbols) with 0 <= j <= 856
Physical layer Physical packet formats For each slot format, a corresponding physical packet format exists: ● short physical packet P00 (96 bits) ● basic physical packet P32 (420 or 424 bits) ● high capacity physical packet P80 (900 or 904 bits) ● low capacity physical packet P00j (100+j or 104+j bits) with 0 <= j <= 856
Physical layer Data fields Physical packets contain multiple data fields with different purposes: ● S-field (synchronization field): 16 bits preamble, 16 bits synchronization word. Contained in all packet formats, used for clock and packet synchronization. ● D-field (data field): contains higher layer data, its size is dependent on the packet format.
Physical layer Data fields ● Z-field: repeats the last 4 bits of the D-field (X- CRC), used to detect unsynchronized interference sliding into the end of the physical packet. The P00- Packet contains no Z-Field, in other packet formats its use is optional and defined by DECT profiles.
Physical layer Physical channels Physical channels provide a connectionless simplex service for data transmission and are created by transmitting physical packets on a particular RF- channel and a particular slot position in successive frames. TDD is used to create double simplex or duplex channels. ● Short physical channel R00: Packet P00 ● Basic physical channel R32: Packet P32 ● High capacity physical channel R80: Packet P80 ● Variable rate physical channel R00j: Packet P00j
Physical layer Timing Besides providing means to transmit and receive packets, the physical layer is also responsible for providing timing to the higher layers. The FP provides timing to all PPs though the S-Field synchronization pulse and the TDMA frame structure. The MAC layer extends the TDMA slot and frame timing by superimposing a multiframe structure on the TDMA frame structure. FPs containing multiple transceivers must have their transceivers synchronized with an accuracy of 15 +- 2us.
Medium Access Control layer Due to the special requirements of a cellular wireless technology, like handling interference, performing various kinds of handover, privacy, combined with the strict timing requirements of a circuit-switched technology and the large number of diverse applications, the DECT MAC layer is the most complicated layer in the DECT stack.
Medium Access Control layer Reference model Internally the MAC layer is split into two parts: ● Cell Site Functions (CSF) include all functions that are concerned with only one cell. Multiple instances of CSF may exist in multi-cell system. These instances would typically be located in different physical locations. ● Cluster Control Functions (CCF) functions are used to control more than one cell. A single instance of CCF exists in a DECT cluster. Communication between CSF and CCF is not specified.
Medium Access Control layer Reference model
Medium Access Control layer Services The MAC layer provides three major groups of services to the higher layers: ● Broadcast message control (BMC): broadcast service for point-to-multipoint communication in the direction FP->PP ● Connectionless message control (CMC): provides bi-directional point-to-point or point-to-multipoint services to the higher layers
Medium Access Control layer Services ● Multi-Bearer control (MBC): provides connection oriented point-to-point service to the higher layers, using one or more traffic bearers. When using multiple bearers, data may be distributed for increased bandwidth, or duplicated for increased reliability.
Medium Access Control layer
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